The present invention pertains to electrostatic fluid dispensing apparatus and more particularly pertains to electrostatic spray nozzles, nozzle assemblies and methods of electrostatic spraying.
In electrostatic fluid dispensing apparatus, a small amount of fluid is electrostatically charged and controllably dispensed in one or more ligaments, jets or streams or paths of droplets or other fluid paths. The term "fluid" is used herein to refer to liquids and to other flowable materials and to other materials made flowable by the applications of heat and/or pressure. The term "fluid paths" is used herein to refer broadly to ligaments, jets and streams and other continuous or discontinuous paths of fluid.
Previous electrostatic spray nozzles with a single point, for production of a single jet, are typically in the form of an electrified capillary, for example Winston, U.S. Pat. No. 3,060,429. In these nozzles, fluid is introduced through a small capillary port, typically about 0.001 inches in diameter, at a pressure which in itself is insufficient to produce flow. By imposing an electric field between the extremity of the nozzle and a conductive, nearby (typically one-quarter inch distant) substrate, a small jet of charged liquid can be forced to fire. Electrodes placed adjacent to the jet's path can impress a field and steer the jet.
This kind of nozzle has a rapid response time and is therefore widely used in high speed printing. These nozzles have limited usefulness in other applications due to the shortcomings of extremely limited throughput flow rates, limited resistivity and viscosity ranges of materials that can be fired, and required close positioning to substrates.
Prior nozzles generally, both single jet and multi-jet, also have a variety of other shortcomings. Nozzles having small orifices tend to clog with foreign matter, are difficult to fabricate in very small diameters and are subject to rapid wear due to local abrasion. Such nozzles may also not be operable with all flowable materials which may range from liquids to gels, from pure materials to suspensions of flowable material and foreign matter, and have a wide range of resistivities and viscosities.
Apparatus using mechanical and aerodynamic dropletization present difficulties as to high energy requirements and overspraying problems.
Another problem for electrostatic dispensing apparatus is the difficulty in providing a high percentage of the theoretical electrostatic charge limit, referred to as the Rayleigh Charge, on droplets or flow paths of a wide range of sizes.
The multi-point nozzle found in Escallon, et al U.S. Pat. No. 4,749,125 obviates the need for small orifices and has found many useful applications in areas as diverse as high speed metal lubrication and placing chemical treatments on foodstuffs or plants. Such nozzles, with the pure direct current power sources, are useful with throughput materials having resistivities down to about 10.sup.6 ohm-centimeters. Below that value, however, a marked decrease of particle number generation occurs, as a few oversized particles dominate the output. Since many useful throughput materials lie below this resistivity, including emulsions, alchohols, glycerins and other water based mixtures, there is a need for a nozzle design whose resistivity limits for good output characteristics are lower. Further, dilute water based mixtures pose additional difficulties due to their high surface tension. Pure water has a surface tension which does not lend itself to proper electrical atomization. The nozzles in Escallon, et al have electrodes that are relatively complex in shape, due to their use as both hydraulic and electrification elements, that must be accurately positioned for proper electrification of less conductive or more resistive throughput materials.
It is therefore highly desirable to provide an improved nozzle, an improved nozzle assembly and an improved method.
It is also highly desirable to provide an improved nozzle, an improved nozzle assembly and an improved method which facilitates the dispensing of controlled amounts of fluid in a single fluid path.
It is also highly desirable to provide an improved nozzle, an improved nozzle assembly and an improved method which do not utilize a small aperture or aerodynamic dropletization.
It is also highly desirable to provide an improved nozzle, an improved nozzle assembly and an improved method, which have a high throughput rate, do not require close positioning to the target and yet provide a uniform and stable output over a broad range of resistivities and viscosities and surface tension.
It is also highly desirable to provide an improved nozzle, an improved nozzle assembly and an improved method in which electrostatic characteristics are such that a high percentage of the theoretical charge limit can be imposed.
It is also highly desirable to provide an improved nozzle, an improved nozzle assembly and an improved method in which uniformly charged droplets of uniform size may be dispensed using direct current at low throughput resistivities and high flow rates.
It is also highly desirable to provide an improved nozzle, an improved nozzle assembly and an improved method in which the nozzle has a simple electrode and dimensional element.
It is also highly desirable to provide an improved nozzle, an improved nozzle assembly and an improved method for dispensing a wide variety of flowable materials ranging from liquids to gels including flowable materials with suspensions of foreign materials.
It is finally highly desirable to provide an improved nozzle, an improved nozzle assembly and an improved method which meet all of the above desired features.